x - the score he must earn
The average of n numbers is the sum of these numbers divided by n.
\(\frac{64+69+73+x}{4}=70 \\
\frac{206+x}{4}=70 \ \ \ \ \ \ \ \ \ \ \ \ |\times 4 \\
206+x=280 \ \ \ \ \ \ \ \ |-206 \\
x=74\)
He must earn a score of (at least) 74.